A seat belt or seatbelt, sometimes called a safety belt, is a safety harness

Safety harness

A safety harness is a form of protective equipment designed to protect a person, animal, or object from injury or damage. The harness is an attachment between a stationary and non-stationary object and is usually fabricated from rope, cable or webbing and locking hardware...

 designed to secure the occupant of a vehicle

Vehicle

A vehicle is a device that is designed or used to transport people or cargo. Most often vehicles are manufactured, such as bicycles, cars, motorcycles, trains, ships, boats, and aircraft....

 against harmful movement that may result from a collision

Collision

A collision is an isolated event which two or more moving bodies exert forces on each other for a relatively short time.Although the most common colloquial use of the word "collision" refers to accidents in which two or more objects collide, the scientific use of the word "collision" implies...

 or a sudden stop. A seat belt reduces the likelihood and severity of injury

Injury

-By cause:*Traumatic injury, a body wound or shock produced by sudden physical injury, as from violence or accident*Other injuries from external physical causes, such as radiation injury, burn injury or frostbite*Injury from infection...

 in a traffic collision by stopping the vehicle occupant from hitting hard interior elements of the vehicle or other passengers (the so-called second impact

Second Impact (safety)

The second impact is the impact suffered between a vehicle occupant and the vehicle he or she is riding in during some kind of safety incident. The first impact is the impact between the vehicle and another object...

), by keeping occupants positioned correctly for maximum benefit from the airbag

Airbag

An Airbag is a vehicle safety device. It is an occupant restraint consisting of a flexible envelope designed to inflate rapidly during an automobile collision, to prevent occupants from striking interior objects such as the steering wheel or a window...

, if the vehicle is so equipped, and by preventing occupants being ejected from the vehicle.